1. Field Invention
This invention relates generally to mobile oil well drilling equipment. More specifically, this invention is related to mobile, self-propelled, workover and completion rigs that are particularly useful for closely spaced wells.
2. Descriotion Of The Related Art
Mobile oil well drilling equipment has been in existence for many years, and facilitates the rig being moved to a well site in a partially assembled state. The mobile rig includes a mast which is folded down and transported on a powered carrier on wheels. At the well site, the rig is erected, for example, with the use of a telescoping mast. A number of supports are extended from the carrier to the ground to support the rig and pipe suspended or hung on the rig. Other components of the rig include pipe handling equipment, power generation equipment and mud equipment, all of which are transported and positioned in close proximity to the drilling rig. An example of a mobile drilling rig is shown in U.S. Pat. No. 4,899,832.
For drilling operations in arctic locations (for example, on the North Slope of Alaska) it is desirable that the drilling rig itself and various components of drilling equipment be maintained in a heated enclosure, as much as possible, for protection of equipment and personnel working on the rig. In arctic conditions, it is critical that equipment and personnel be shielded from sub-zero temperatures, snow and ice that impairs drilling work. The equipment which must be protected includes the drilling rig itself, the mud equipment, and pipe storage and handling equipment. The mud equipment includes all devices for mixing and controlling mud flow to the rig. For pipe storage and pipe handling, it is necessary to warm, clean and store several thousands of feet of pipe, in 30 to 45 foot lengths prior to conveying the pipe to the mast on the drilling rig.
Environmental sensitivity is another concern at arctic locations such as oil fields on the North Slope of Alaska. It is critical that wells and mobile equipment be installed and operated to minimize any danger or risk to the environment.
Typically, it is necessary to drill several thousand feet into permafrost. To protect the permafrost and surrounding tundra from melting, a five foot deep gravel pad is laid out on top of the tundra. The gravel pad provides an insulating surface above the permafrost or tundra, and the pad provides a surface on which the mobile drilling equipment travels. The gravel pad provides a stable surface for the rig over the permafrost or tundra when the surrounding permafrost thaws. The gravel serves as an insulating layer over the permafrost. Each gravel pad accommodates several dozen wells, and is on the order of one half mile in length and width. The wells, drilling equipment, sump areas, and other equipment are located on the gravel pad. A flow line or series of flow lines extends from each well to a sump or reserve pit. Typically, clusters of wells are located along one or more centerlines on the gravel pad. For example, each well center may be located approximately one hundred feet from the adjacent well center. The 100 foot distance between the well centers was considered to be necessary to prevent the radiant heat from each well from thawing the permafrost. In recent years, however, it has been found that wells may be positioned on well centers closer than 100 feet without the radiant heat problem.
In an effort to minimize environmental impact on tundra and wildlife, additional wells are drilled on existing gravel pads between wells previously in existence, rather than building additional sites. With these additional wells, ultimately each well is closely spaced to the adjacent wells. For example, each well center may be less than fifty feet from the adjacent well center, and in some cases as close as ten feet. The operating space is further restricted because each well is enclosed in a well house. Typically, the well house base is sixteen feet by fourteen feet, with a height of eighteen feet. The present invention is particularly useful for workover and completion of well centers that are as close as thirty feet.
The close proximity of each well to adjacent wells presents a problem for mobile workover and completion rigs. A mobile rig must be maneuvered into position and operated without interfering with adjacent wells. It is extremely undesirable to shut in the adjacent wells. Another limitation on the maneuvering and operating space for the rig is the flow line between each well and the sump or reserve pit. It is not possible to position the rig on the flow line side of the wells.
In the past, the problem of workover and completion of closely spaced well centers has been addressed by two alternative approaches: (a) individualized set-up and assembly of components in the space available at each well, or (b) cantilever rig design requiring significantly strengthened structural reinforcement.
Alternative (a) is undesirable because the equipment must be set up in a specific arrangement at each well, then disassembled, at considerable time and expense. Therefore, alternative (a) is a particular problem for workover and completion under arctic conditions.
Alternative (b), cantilever rigs, may be used for workover and completion of wells on as close as ten foot well centers. Cantilever rigs are designed to back up at a 90 degree angle to the line of wells, and have a U-shape to position the rig over the top of the well. However, cantilever rigs require significantly strengthened structural reinforcement at greatly added expense over other types of rigs. The structural reinforcement is required because a mast bearing 500,000 pounds or more of load must be cantilevered over the well center, rather than supported on the ground.
A general object of the present invention is to provide a non-cantilevered configuration of mobile drilling apparatus that may be used in arctic conditions on wells having closely spaced well centers.
Another object of the present invention is to provide a new and improved method and apparatus for positioning a mobile workover and completion rig between adjacent well houses.
A further object of the present invention is to provide a highly portable enclosed workover and completion rig.
Another object of the present invention is to provide a mobile rig and method which eliminates the need to shut down adjacent wells during workover and completion operations.
Another object of the present invention is to provide a new and improved modular configuration for mobile oil well workover and completion equipment capable of working between adjacent wells having thirty foot well centers.